Open Access

Etching characteristics of Si{110} in 20 wt% KOH with addition of hydroxylamine for the fabrication of bulk micromachined MEMS

Micro and Nano Systems Letters20175:23

https://doi.org/10.1186/s40486-017-0057-7

Received: 1 March 2017

Accepted: 23 May 2017

Published: 26 May 2017

Abstract

Anisotropic wet etching is a most widely employed for the fabrication of MEMS/NEMS structures using silicon bulk micromachining. The use of Si{110} in MEMS is inevitable when a microstructure with vertical sidewall is to be fabricated using wet anisotropic etching. In most commonly employed etchants (i.e. TMAH and KOH), potassium hydroxide (KOH) exhibits higher etch rate and provides improved anisotropy between Si{111} and Si{110} planes. In the manufacturing company, high etch rate is demanded to increase the productivity that eventually reduces the cost of end product. In order to modify the etching characteristics of KOH for the micromachining of Si{110}, we have investigated the effect of hydroxylamine (NH2OH) in 20 wt% KOH solution. The concentration of NH2OH is varied from 0 to 20% and the etching is carried out at 75 °C. The etching characteristics which are studied in this work includes the etch rates of Si{110} and silicon dioxide, etched surface morphology, and undercutting at convex corners. The etch rate of Si{110} in 20 wt% KOH + 15% NH2OH solution is measured to be four times more than that of pure 20 wt% KOH. Moreover, the addition of NH2OH increases the undercutting at convex corners and enhances the etch selectivity between Si and SiO2.

Keywords

Wet anisotropic etching Bulk micromachining Silicon Corner undercutting KOH MEMS Hydroxylamine (NH2OH) TMAH Si{110} Etching characteristics

Background

Wet anisotropic etching is a main process of silicon bulk micromachining for the fabrication of different types of microstructures (e.g. cantilever, diaphragm, cavity, etc.) [15]. It is a low cost technique and suitable for batch process. Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are the two main etchants used for wet anisotropic etching-based silicon bulk micromachining [612]. These etchants are thoroughly investigated under various etching conditions. KOH is preferred over TMAH to achieve high etch selectivity between {111} and {110}/{100} planes. In the fabrication of microstructure using etching process, etch rate is a key parameter in manufacturing as it influences production rate which eventually affects the cost of final product. Mostly high etch rate is desirable to reduce production cost. An etchant with high etch rate may also provide high etch selectivity between silicon and mask layer, which is very useful to use same thickness mask layer for prolonged etching. The important parameters which affect the etch rate and surface morphology are etchant concentration, etching temperature, ultrasonication/microwave irradiation during etching, and the addition of different kinds of additives to etchant. Each method has its own benefits and drawbacks. Maximum etch rate of Si{100} and Si{110} is obtained in KOH with a concentration range from 15 to 25 wt% [811]. Different kinds of additives (e.g. redoxsystem or complexants, oxidizing agent, surfactants, and metal impurities) [1317] are added into KOH to get high speed etching and the alcohols/surfactants are incorporated to improve the surface morphology [1824]. Moreover, etching at the boiling point of the etchant [11, 25], microwave irradiation of the etchant [26], ultrasonic agitation of the etchant [27] have been employed to increase the etch rate. The ultrasonic method may rupture the fragile structures and microwave irradiation technique causes irradiation damage to the structures. Anisotropic etching at very high temperature (e.g. boiling point of etchant) is not preferable as it affects the anisotropy and the etch selectivity between SiO2 and silicon. Therefore, it is required to improve the etching characteristics by adding some additives.

In all kinds of wet anisotropic etchants, Si{111} planes exhibit minimum etch rate. If the mask edges are aligned along the directions comprises {111} planes, wet anisotropic etching provides microstructures with smooth sidewalls due to the emergence of {111} planes at these directions. The angle between sidewall and wafer surface depends on the wafer orientation. Moreover, the number of directions along which {111} planes appear depend on the orientation of wafer surface. In the case of {100} wafer, four {111} planes making an angle of 54.7° with wafer surface expose at 110 directions. Hence {100} wafer is suitable to fabricate rectangular shaped cavities or suspended structures over rectangular shape cavity using wet anisotropic etching [3, 6, 12, 21]. In the case of the wafer with {110} surface, two slanted planes making an angle of 35.5° with wafer surface and four vertical planes with respect to wafer surface appear along 110 and 112 directions, respectively. Therefore, in order to fabricate microstructures with vertical sidewalls {110} wafer is a most appropriate choice [10, 2834]. It can be used to fabricate deep channels/cavities with vertical sidewall.

In this paper, we have studied the etching characteristics of {110}-oriented silicon wafer in 20 wt% KOH solution without and with addition of NH2OH. It is mainly focused to investigate the effect of NH2OH in 20 wt% KOH to achieve improved etching characteristics for applications in silicon bulk micromachining for the formation of MEMS structures.

Experimental details

In this work, 4-in. diameter {110}-oriented p-type doped Czochralski-grown silicon wafers with 5–10 Ω-cm resistivity are used. One micron thick oxide layer grown by thermal oxidation process has been used as mask to protect unwanted places from etching. Photolithography method is used to pattern the thermal oxide layer. After transferring the mask pattern on photoresist layer, oxide etching is employed in buffered hydrofluoric acid (BHF). Thereafter, wafer is rinsed in DI water followed by removal of photoresist using acetone. After patterning of oxide layer, wafer is diced into small chips. 20 wt% KOH (99.99%, Alfa Aesar) is used as main etchant, while hydroxylamine solution (NH2OH) is used as an additive solution. In order to investigate the effect of NH2OH on the etching characteristics of KOH, various concentrations of NH2OH (5, 10, 15 and 20%) added into KOH solution. Experiments are performed in one litre etchant at 75 ± 1 °C. In order to maintain the constant temperature of the etchant during etching process, a constant-temperature water bath is used. Teflon made cylindrical container equipped with reflux condenser is used to avoid change in etchant concentration due to evaporation. PFA made chip holder with multiple slots is used to hold samples in order to etch many samples at a time under same etching conditions. Before starting experiment, samples are cleaned in piranha bath (1:1; H2O2:H2SO4) to remove organic impurities. This step is followed by thorough rinse in DI water. The presence of very thin oxide layer (i.e. native oxide) delay silicon etching. Therefore, prior to dipping the samples into etchant, the samples are immersed in 1% HF for 30 s followed by DI water rinse. After silicon etching process, etch depth, surface roughness and undercutting are measured using 3D measuring laser microscope (Olympus, OLS4000). Moreover scanning electron microscope (SEM) is employed to inspect etched surface morphology. The thickness of oxide layer after different times of etching is measured using ellipsometry. In this work, mask patterns with simple shapes such as cantilever, parallelogram/rhombus shapes are used to determine different etching characteristics, e.g., etch rate, surface morphology, undercutting at convex corners. Rhombus shape geometries (mesa and opening) are used to measure the etch rate, surface morphology, and undercutting at convex corners, while cantilever patterns are used to demonstrate the application of proposed etchant for the fabrication of suspended MEMS structures. The dimensions of the parallelograms and cantilever shape patterns vary from 300 µm × 300 µm to 1000 µm × 1000 µm and 50 µm × 100 µm to 200 µm × 400 µm, respectively.

Results and discussion

Etching characteristics of Si{110} including etch rate, surface roughness/morphology, and undercutting at convex corners are studied on the samples etched in pure and NH2OH-added 20 wt% KOH. To study etching characteristics, the concentration of NH2OH is varied from 5 to 20% in steps of 5%. Detailed descriptions of these characteristics are presented in following subsections.

Etch rate

Etch rates of Si{110} in pure and various concentrations of hydroxylamine (NH2OH) added 20 wt% KOH solution are presented in Fig. 1. The etch rate increases with increase of NH2OH concentration up to 15% and starts decreasing if the NH2OH concentration is further increased. The etch rate in 15% NH2OH-added KOH is four times more than that in pure 20 wt% KOH. The standard deviation indicated by error bars is calculated by taking six measurements on the same sample at different locations. The mechanism behind the increase of etch rate in NH2OH-added KOH may be interesting finding, but it is not the main focus of the present work. The major objective of this work is to investigate the effect of NH2OH to achieve high etch rate and undercutting to promote the application of wet anisotropic etching in MEMS fabrication. In several studies, it has been claimed that the etch rate of an etchant increases if its wettability improves such as the wettability of KOH increases when anionic additives are added [35, 36]. In the case of NH2OH-added KOH solution, NH2OH and OH ions participate in the reaction to forms NH2O ions and water until it reaches in chemical equilibrium [37]. NH2O, OH and water are active species in NH2OH-added alkaline solution. The etching mechanism in KOH solution is well known. First step of etching process is the chemical oxidation in which hydrogen-terminated silicon (Si–H) becomes a hydroxyl-terminated silicon atom (Si–OH) [38]. It is a slow process. We speculate that in NH2OH-added KOH solution H of Si–H is replaced by NH2O. After becoming OH or NH2O terminated, the Si–Si backbonds exhibit significant polarity due to the large electronegativity of O that results in weakening of the backbonds which are easily attacked by the polar water molecules, leading to the removal of silicon atom as a Si(OH)4 product. Due to the more electronegativity of oxygen in NH2O than in OH, the polarization and weakening of the backbonds in Si-ONH2 is more prominent. Thus silicon backbonds are speedily hydrolysed (or attacked by water) in NH2OH-added KOH. In addition to H2O, NH2O and OH ions, other intermediate compounds of self-decomposed NH2OH in the solution may also participate in the reaction to dissolve silicon [37, 39]. Thus etching in NH2OH-added KOH proceeds at a faster rate in comparison to that in pure KOH. Increasing concentration of NH2OH above 15% decreases etch rate. It may be because of the decrease in NH2O and OH ions concentration and/or decrease in freely available water due to the formation of clusters (K+, OH, NH2O, etc.) [40, 41].
Fig. 1

Etch rate of Si{110} in pure and various concentrations of NH2OH-added 20 wt% KOH solution at 75 °C

Etch selectivity with thermal oxide

Silicon dioxide is extensively used in silicon micromachining as etch mask layer to create various kinds of grooves and cavities. Moreover, it is employed as structural layer for the fabrication of MEMS structures such as cantilever. It can easily be grown by various techniques such as thermal oxidation, chemical vapor deposition (CVD), and anodic oxidation [42, 43]. However thermal oxidation is most widely used as it provides excellent quality oxide layer and interface. Oxide layer is usually patterned by photolithography followed by oxide etching in HF/BHF. The etch rate is calculated by measuring the oxide thickness at different locations on the same sample after different times of etching. Figure 2 shows the etch rate of SiO2 in 20 wt% KOH with varying concentration of NH2OH. It can be noticed that the oxide etch rate increases with increase of NH2OH concentration. In the fabrication of MEMS structures using silicon wet anisotropic etching based bulk micromachining, the etch selectivity between silicon and mask/structural layer (e.g. SiO2) is an important concern. It is defined as the ratio of the etch rate of silicon to that of SiO2. If an etchant provides high etch selectivity, mask/structural layer can be exposed in the etchant for a longer time, which is needed to fabricate deep cavities/grooves or freestanding structures (e.g. cantilever) on silicon wafer. The etch selectively between Si{110} and SiO2 calculated using the results of Figs. 1 and 2 is presented in Fig. 3. It can easily be noticed in Fig. 3 that the etch selectivity increases significantly with increase of NH2OH concentration. In other words, we can say that the addition of NH2OH to KOH solution considerably improves the etch selectivity between Si{110} and SiO2. Hence it can be concluded that the same thickness oxide layer in NH2OH-added KOH can be used to form larger depth cavities and grooves in comparison to pure KOH. In addition to that oxide layer can be used as structural layer to fabricate freestanding structures using NH2OH-added KOH as anisotropic etchant.
Fig. 2

Etch rate of thermal silicon dioxide (SiO2) in pure and different concentrations of NH2OH-added 20 wt% KOH at 75 °C

Fig. 3

Etch rate selectivity between Si{110} and silicon dioxide (Si{110}/SiO2) in pure and NH2OH-added 20 wt% KOH at 75 °C

Etched surface morphology

Surface morphology is one of major concerns in optical MEMS applications and designing high-efficiency solar cell, etc. Etched surface morphology primarily depends on the etchant type, etchant concentration, etching temperature, additives, and agitation of etchant during etching process. The average etched surface roughness decreases with the increase of etching temperature and KOH concentration [11, 25, 44, 45]. Surface roughness of the samples etched in 20 wt% KOH solution without and with addition of various concentrations of NH2OH is measured using 3D measuring laser microscope (Olympus, OLS4000) and presented in Fig. 4. The standard deviation indicated by error bars is calculated by taking six measurements on the same sample at different locations. Figure 5 presents SEM images of the etched samples corresponding to surface roughness shown in Fig. 4. Average surface roughness (Ra) of the samples etched in NH2OH-added 20 wt% KOH solution is nearly same as those are etched in pure 20 wt% KOH. It means that the etched surface of Si{110} is not affected significantly when NH2OH is added into KOH solution. Main cause of surface roughness in the wet etching process is micromasking by the hydrogen bubbles and/or impurities on the surface during the etching process [24, 4446].
Fig. 4

Average surface roughness (Ra) in 20 wt% KOH solution without and with addition of NH2OH solutions at 75 °C

Fig. 5

SEM photographs showing the surface morphology of the samples etched in 20 wt% KOH solution without and with addition of NH2OH at 75 °C

Undercutting at convex corner

In silicon wet anisotropic etching, the corners formed by the intersection of {111} planes are termed as concave and convex corners. These corners are produced by the intersection of 110 and 112 directions on Si{110} surface as these directions contain {111} planes which expose during wet anisotropic etching. Although both types of corners (concave and convex) are shaped by the intersection of {111} planes, they have opposite etching characteristics. Concave corners do not encounter any kind of undercutting, while convex corners face severe undercutting, depending on the type of etchant, in all kinds of alkaline solutions [4652]. Si{110} wafer is a primary choice when the microstructures with vertical sidewalls formed by {111} planes are fabricated using wet anisotropic etching [2834, 52]. These vertical sidewalls appear at 112 directions which form a rhombus shape structure containing two types of convex corners (acute and obtuse corners) as presented in Fig. 6. Undercutting rate (undercutting length along 112 direction/etch time) and undercutting ratio (undercutting length along 112 direction/etch depth) at both types of convex corners as a function of NH2OH concentration are presented in Fig. 6. Undercutting rate increases as the concentration of NH2OH increases up to 15% NH2OH and is around four times more than that in pure 20 wt% KOH. The undercutting at convex corners takes place mainly due to the emergence of high index planes [29, 4652]. The main reason behind the increase in undercutting is the increase of the etch rate of high index planes appearing at convex corners during etching process.
Fig. 6

Undercutting at acute and obtuse corners of a rhombus shape structure formed by 112 directions on Si{110} wafer in pure and various concentrations of NH2OH-added 20 wt% KOH solution: undercutting rate (l/t) at a acute and b obtuse corners; undercutting ratio (l/d) at c acute and d obtuse corners

In the fabrication of suspended MEMS components (e.g. cantilever beams), underneath material is removed by undercutting process. Hence high undercutting is desirable for the fast removal of underneath material to release the microstructures. In the present research NH2OH-added KOH provides high undercutting at convex corner and therefore it is very useful for the fabrication of freestanding structures. In addition to that it exhibits high etch selectivity between Si{110} and SiO2 as presented in Fig. 3 and explained in “Etch selectivity with thermal oxide”, which is required for the fabrication of SiO2 microstructures. To demonstrate the application of high undercutting for the realization of suspended structures for MEMS, silicon dioxide cantilever beams are fabricated in 15% NH2OH-added 20 wt% KOH solution. Figure 7a, b present the schematic views of patterned oxide layer on Si{110} and released cantilever beam after anisotropic etching, respectively. Optical and SEM images of freestanding cantilever beam are presented in Fig. 7d, e, respectively. The fabrication of oxide cantilever beam indicates that the NH2OH-added KOH exhibits high etch selectivity between silicon and silicon dioxide. Pure KOH provides very low etch selectivity between Si{110} and SiO2 in comparison to NH2OH-added KOH as presented in Fig. 3. Due to this reason, oxide layer cannot survive for a longer time in pure KOH solution and therefore it cannot be used as structural layer for the fabrication of freestanding structure. Hence we can say that NH2OH-added KOH is a suitable wet anisotropic etchant for the fabrication of SiO2 microstructures.
Fig. 7

Silicon dioxide cantilever beam fabricated in NH2OH-added KOH solution: a patterning of oxide layer using photolithography and oxide etching, b optical image of cantilever beam patterned in oxide layer, c wet anisotropic etching to release the beam, d optical and e SEM images of released cantilevers

Conclusions

Etching characteristics of Si{110} surface in pure and different concentration of NH2OH-added 20 wt% KOH are studied for applications in silicon wet bulk micromachining. The etching characteristics of KOH solution are changed drastically when NH2OH is added. The etch rate and undercutting are improved significantly. High etch rate is very useful to achieve larger etch depth in less time in comparison to common etchant. Increase in undercutting at convex corner is beneficial for the fast release of the microstructures. Both these characteristics are indispensable for reducing etch time and therefore useful for industries to increase the productivity. Moreover, NH2OH-added KOH provides high etch selectivity between silicon and oxide (i.e. Si/SiO2) in comparison to pure KOH. High etch selectivity can be exploited for the fabrication of MEMS structures using silicon dioxide as mask/structural layer. It can be concluded that the results presented in this paper are highly useful for research and industrial applications.

Declarations

Authors’ contributions

AVNR and VS did experiments. AVNR and PP wrote the manuscript. All authors read and approved the final manuscript.

Acknowledgements

Sincere thanks to Prof. K. Sato, Aichi Institute of Technology Toyota, Japan and Mr. G. Raju, Research Scholar at Department of Chemistry, Indian Institute of Technology Hyderabad for their suggestions.

Competing interests

The authors declare that they have no competing interests.

Funding

This work was supported by research grant from the Department of Science and Technology (Project No. SR/S3/MERC/072/2011) and the Council of Scientific and Industrial Research (CSIR, Ref: 03(1320)/14/EMR-II), New Delhi, India.

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Authors’ Affiliations

(1)
MEMS and Micro/Nano Systems Laboratory, Department of Physics, Indian Institute of Technology Hyderabad

References

  1. Zubel I, Kramkowska M (2005) Possibilities of extension of 3D shapes by bulk micromachining of different Si (hkl) substrates. J Micromech Microeng 15(3):485–493View ArticleGoogle Scholar
  2. Yang EH, Yang SS, Han SW, Kim SY (2005) Fabrication and dynamic testing of electrostatic actuators with p+ silicon diaphragms. Sens Actuators A phys 50:151–156View ArticleGoogle Scholar
  3. Pal P, Sato K (2009) Complex three dimensional structures in Si{100} using wet bulk micromachining. J Micromech Microeng 19(10):105008View ArticleGoogle Scholar
  4. Xu YW, Michael A, Kwok CY (2011) Formation of ultra-smooth 45 micromirror on (100) silicon with low concentration TMAH and surfactant: techniques for enlarging the truly 45 portion. Sens Actuators A Phys 166(1):164–171View ArticleGoogle Scholar
  5. Lee S, Park S, Cho D (1999) The surface/bulk micromachining (SBM) process: a new method for fabricating released microelectromechanical systems in single crystal silicon. J Microelectromech Syst 8:409–416View ArticleGoogle Scholar
  6. Pal P, Gosalvez MA, Sato K (2010) Silicon micromachining based on surfactant-added tetramethyl ammonium hydroxide: etching mechanism and advanced application. Jpn J Appl Phys 49:056702View ArticleGoogle Scholar
  7. Gosalvez MA, Pal P, Ferrando N, Hida H, Sato K (2001) Experimental procurement of the complete 3D etch rate distribution of Si in anisotropic etchants based on vertically micromachined wagon wheel samples. J Micromech Microeng 21(12):125007View ArticleGoogle Scholar
  8. Sato K, Shikida M, Matsushima Y, Yamashiro T, Asaumi K, Iriye Y, Yamamoto M (1998) Characterization of orientation-dependent etching properties of single-crystal silicon: effects of KOH concentration. Sens Actuators A Phys 61(1):87–93View ArticleGoogle Scholar
  9. Seidel H, Csepregi L, Heuberger A, Baumgärtel H (1990) Anisotropic etching of crystalline silicon in alkaline solutions I. Orientation dependence and behavior of passivation layers. J Electrochem Soc 137(11):3612–3626View ArticleGoogle Scholar
  10. Dutta S, Imran M, Kumar P, Pal R, Datta P, Chatterjee R (2011) Comparison of etch characteristics of KOH, TMAH and EDP for bulk micromachining of silicon (110). Microsyst Technol 17(10–11):1621–1628View ArticleGoogle Scholar
  11. Tanaka H, Yamashita S, Abe Y, Shikida M, Sato K (2004) Fast etching of silicon with a smooth surface in high temperature ranges near the boiling point of KOH solution. Sens Actuators A Phys 114(2):516–520View ArticleGoogle Scholar
  12. Pal P, Sato K (2010) Fabrication methods based on wet etching process for the realization of silicon MEMS structures with new shapes. Microsyst Technol 16(7):1165–1174View ArticleGoogle Scholar
  13. Moldovan C, Iosub R, Dascalu D, Nechifor G (1999) Anisotropic etching of silicon in a complexant redox alkaline system. Sens Actuators B Chem 58(1):438–449View ArticleGoogle Scholar
  14. Van den Meerakker JE (1990) The reduction of hydrogen peroxide at silicon in weak alkaline solutions. Electrochim Acta 35(8):1267–1272View ArticleGoogle Scholar
  15. Sotoaka R (2008) New etchants for high speed anisotropic etching of silicon. J Surf Finish Soc Jpn 59(2):104–106View ArticleGoogle Scholar
  16. Yang CR, Chen PY, Yang CH, Chiou YC, Lee RT (2005) Effects of various ion-typed surfactants on silicon anisotropic etching properties in KOH and TMAH solutions. Sens Actuators A Phys 119(1):271–281View ArticleGoogle Scholar
  17. Hein A, Dorsch O, Obermeier E (1997) Effects of metallic impurities on anisotropic etching of silicon in aqueous KOH-solutions. In: Solid state sensors and actuators, TRANSDUCERS’97 (Chicago, U.S). IEEE, New York, pp 687–690Google Scholar
  18. Zubel I, Kramkowska M (2002) The effect of alcohol additives on etching characteristics in KOH solutions. Sens Actuators A Phys 101(3):255–261View ArticleGoogle Scholar
  19. Sundaram KB, Vijayakumar A, Subramanian G (2005) Smooth etching of silicon using TMAH and isopropyl alcohol for MEMS applications. Microelectron Eng 77(3):230–241View ArticleGoogle Scholar
  20. Merlos A, Acero M, Bao MH, Bausells J, Esteve J (1993) TMAH/IPA anisotropic etching characteristics. Sens Actuators A Phys 37:737–743View ArticleGoogle Scholar
  21. Ashok A, Pal P (2015) Silicon micromachining in 25 wt% TMAH without and with surfactant concentrations ranging from ppb to ppm. Microsyst Technol 23(1):47–54View ArticleGoogle Scholar
  22. Pal P, Sato K, Gosalvez MA, Tang B, Hida H, Shikida M (2010) Fabrication of novel microstructures based on orientation dependent adsorption of surfactant molecules in TMAH solution. J Micromech Microeng 21(1):015008View ArticleGoogle Scholar
  23. Resnik D, Vrtacnik D, Aljancic U, Mozek M, Amon S (2005) The role of triton surfactant in anisotropic etching of 110 reflective planes on (100) silicon. J Micromech Microeng 15(6):1174–1183View ArticleGoogle Scholar
  24. Cheng D, Gosálvez MA, Hori T, Sato K, Shikida M (2006) Improvement in smoothness of anisotropically etched silicon surfaces: effects of surfactant and TMAH concentrations. Sens Actuators A Phys 125(2):415–421View ArticleGoogle Scholar
  25. Tang B, Sato K, Zhang D, Cheng Y (2014) Fast Si(100) etching with a smooth surface near the boiling temperature in surfactant-modified tetramethylammonium hydroxide solutions. Micro Nano Lett 9(9):582–584View ArticleGoogle Scholar
  26. Dziuban JA, Walczak R (2001) EMSi-microwave enhanced fast deep anisotropic etching of silicon for MEMS. Sens Mater 13(1):41–55Google Scholar
  27. Chen J, Liu L, Li Z, Tan Z, Jiang Q, Fang H, Xu Y, Liu Y (2002) Study of anisotropic etching of (100) Si with ultrasonic agitation. Sens Actuators A 96(2):152–156View ArticleGoogle Scholar
  28. Kolesar ES Jr, Carver MW (1989) Deep anisotropic etching of tapered channels in (110)-oriented silicon. Chem Mater 1(6):634–639View ArticleGoogle Scholar
  29. Pal P, Gosalvez MA, Sato K, Hida H, Xing Y (2014) Anisotropic etching on Si{110}: experiment and simulation for the formation of microstructures with convex corners. J Micromech Microeng 24(12):125001View ArticleGoogle Scholar
  30. Lee D, Yu K, Krishnamoorthy U, Solgaard O (2009) Vertical mirror fabrication combining KOH etch and DRIE of (110) silicon. J Microelectromech Syst 18(1):217–227View ArticleGoogle Scholar
  31. Holke A, Henderson HT (1999) Ultra-deep anisotropic etching of (110) silicon. J Micromech Microeng 9(1):51–57View ArticleGoogle Scholar
  32. Uenishi Y, Tsugai M, Mehregany M (1995) Micro-opto-mechanical devices fabricated by anisotropic etching of (110) Silicon. J Micromech Microeng 5(4):305–312View ArticleGoogle Scholar
  33. Kim SH, Lee SH, Lim HT, Kim YK, Lee SK (1997) Anisotropic bulk etching of (110) Silicon with high aspect ratio. IEEJ Trans Sens Micromach 118(1):32–36View ArticleGoogle Scholar
  34. Krause P, Obermeier E (1995) Etch rate and surface roughness of deep narrow U-grooves in (110)-oriented silicon. J Micromech Microeng 5(2):112–114View ArticleGoogle Scholar
  35. Yang CR, Chen PY, Chiou YC, Lee RT (2005) Effects of mechanical agitation and surfactant additive on silicon anisotropic etching in alkaline KOH solution. Sens Actuators A Phys 119(1):263–270View ArticleGoogle Scholar
  36. Divan R, Moldovan N, Camon H (1999) Roughning and smoothing dynamics during KOH silicon etching. Sens Actuators A Phys 74(1):18–23View ArticleGoogle Scholar
  37. Wei C, Saraf SR, Rogers WJ, Mannan MS (2004) Thermal runaway reaction hazards and mechanisms of hydroxylamine with acid/base contaminants. Thermochim Acta 421(1):1–9View ArticleGoogle Scholar
  38. Gosálvez MA, Zubel I, Viinikka E (2015) Wet etching of silicon in handbook of silicon based MEMS materials and technologies, 2nd ed. William Andrew Publishing, Espoo, pp 470–502Google Scholar
  39. Luňák S, Vepřek-Šiška J (1974) The catalytic effect of cations on the decomposition of alkaline solutions of hydroxylamine. Collect Czech Chem Commun 39(2):391–395View ArticleGoogle Scholar
  40. Alcami M, Mo O, Yanez M (1991) Ab initio molecular orbital treatment of hydroxylamine-X+-water and hydroxylamine-X+-ammonia (X = H, Li) clusters. Chem Phys 151(1):21–36View ArticleGoogle Scholar
  41. Vizoso S, Rode BM (1995) The structure of hydroxylamine–water mixtures. Z Naturforsch A Phys Sci 50(2–3):263–273Google Scholar
  42. Pal P, Sato K (2017) Silicon wet bulk micromachining for MEMS. Pan Stanford Publishing, SingaporeGoogle Scholar
  43. Ashok A, Pal P (2014) Investigation of anodic silicon dioxide thin films for MEMS applications. Micro Nano Lett 9(12):830–834View ArticleGoogle Scholar
  44. Palik ED, Glembocki OJ, Heard I Jr, Burno PS, Tenerz L (1991) Etching roughness for (100) silicon surfaces in aqueous KOH. J Appl Phys 70(6):3291–3300View ArticleGoogle Scholar
  45. Van Veenendaal E, Sato K, Shikida M, Van Suchtelen J (2001) Micromorphology of single crystalline silicon surfaces during anisotropic wet chemical etching in KOH and TMAH. Sens Actuators A Phys 93(3):219–231View ArticleGoogle Scholar
  46. Pal P, Sato K, Gosalvez MA, Shikida M (2007) Study of rounded concave and sharp edge convex corners undercutting in CMOS compatible anisotropic etchants. J Micromech Microeng 17(11):2299–2307View ArticleGoogle Scholar
  47. Kim B, Cho DI (1998) Aqueous KOH etching of silicon (110) etch characteristics and compensation methods for convex corners. J Electrochem Soc 145:2499–2508View ArticleGoogle Scholar
  48. Pal P, Singh SS (2013) A simple and robust model to explain convex corner undercutting in wet bulk micromachining. Micro and Nano Syst Lett 1(1):1–6View ArticleGoogle Scholar
  49. Pal P, Sato K, Shikida M, Gosalvez MA (2009) Study of corner compensating structures and fabrication of various shapes of MEMS structures in pure and surfactant added TMAH. Sens Actuators A Phys 154(2):192–203View ArticleGoogle Scholar
  50. Dong W, Zhang X, Liu C, Li M, Xu B, Chen W (2004) Mechanism for convex corner undercutting of (110) silicon in KOH. Microelectron J 35(5):417–419View ArticleGoogle Scholar
  51. Pal P, Sato K (2015) A comprehensive review on convex and concave corners in silicon bulk micromachining based on anisotropic wet chemical etching. Micro Nano Syst Lett 3(6):1–42Google Scholar
  52. Pal P, Singh SS (2013) A new model for the etching characteristics of corners formed by Si{111} planes on Si{110} wafer surface. Engineering 5(11A):1–8View ArticleGoogle Scholar

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